Spectroscopy refers to the study and measurement of light interaction with matter as a function of wavelength. A spectroscopic device generally includes a light source, a wavelength separating or scanning module, a sample cell, and a light detector. Absorption spectroscopy is a technique for identifying the chemical composition of a sample by measuring the absorption of light (or other electromagnetic radiation) passing through the sample as a function of wavelength (or frequency). Absorption spectroscopy is an inherently relative method in that it involves obtaining a spectrum (e.g., a wavelength-dependent signal) of a given sample as well as the spectrum of a known reference, then determining the ratio, difference, or other comparison of the reference spectrum and sample spectrum. In gas phase infrared measurement, the reference or zero beam is often acquired with nitrogen in the sample cell, because nitrogen is an inert gas that does not have any infrared absorption. Depending on the application, another reference or zero gas may be used such as air, argon, helium, etc. A sample spectrum (e.g., signal determined over a range of wavelengths, or at a given set of discrete wavelengths) is acquired with the sample cell filled with the sample being measured. The absorption spectrum can then be computed as the ratio of the sample beam and the zero beam. Generally, the logarithm of the ratio is used to provide linearization of the absorption signal with respect to the density of the molecules or the concentration.
Providing an acceptable reference or zero sample can be problematic due to various technical and/or practical reasons. For example, in the case of a high-purity trace monitoring application, the reference or ‘zero’ needs to be orders of magnitude purer than the target detection limit of the measurement—this may not be commercially or practically available. In other cases, such as in remote stand-alone deployment, maintenance and support for such reference sample is difficult.
Thus, there is a need for spectroscopic systems and methods that address these practical limitations.